Black cat turning grey?

I was just wondering if this is normal.. She's 11 and is currently on innova evo, but I'm most likely switching to the regular innova tomorrow. I didn't notice her turning grey until my sister shaved her today (my cat loves to be shaved, didn't even need to ace her). Now all her short hair on her body is greyish, when she's always been a black/brown with red highlights.

She had a senior panel about 3 weeks ago and everything was good, except her massive girth

Depending on the time of year my Persian Simon will go from smokey black to grey and even a reddish tint. We shave him as well and it's more noticeable as his fur starts growing back. We have always attributed it to the seasons and climate.

The grey you're seeing now is most likely the normal color of your cat's undercoat or roots which is visible because her top coat of black hair has been removed. It's not unusual for solid color cats to have a paler colored undercoat. My solid black kitty also had grey colored undercoat hair.